Studies on true dissimilatory nitrate reduction

Summary

A detailed study of the conditions of NO formation byThiobacillus suspensions led to the conclusion that NO occurs only under circumstances that are decidedly adverse to normal metabolism. The presence of otherwise toxic concentrations of nitrite at pH 6.4, of cyanide or of a combination of both can be mentioned.

A striking observation was that even at pH 7.2 “physiologically old” cells did show NO formation, but then also only in the presence of nitrite and cyanide.

It could be shown that the thiosulphate oxidizing bacteriaMicrococcus denitrificans andPseudomonas stutzeri evolved no NO under conditions in whichT. denitrificans did.

The typical influence of the “physiological age” of the cells in the experiments withT. denitrificans suggested the possibility of NO production with “old” cells ofPs. aeruginosa grown on nitrate and glucose.

With these “physiologically old” bacteria NO production could indeed be obtained. The adverse conditions, already so characteristic in the experiments withT. denitrificans, were still more pronounced in this case. Here even the combined conditions of low pH (6.4), presence of nitrite and cyanide and “physiologically old” cells were necessary for the production of this gas.

The consumption of NO byPseudomonas stutzeri andMicrococcus denitrificans is established. However, suspensions of the non denitrifying microorganismsPseudomonas fluorescens andSaccharomyces cerevisiae showed also NO consumption when cultivated aerobically in the absence of nitrate. The significance of NO production and consumption by microorganisms is discussed, together with some observations on the behaviour of isolated bacterial cytochrome towards NO.